1. Field of the Invention
The present invention relates generally to methods and apparatus for determining the shape of a flexible body, and more particularly to such methods and apparatus which utilize Bragg grating sensors, time, spatial and wave division multiplexing and strain-to-shape structural analysis to resolve the shape of such flexible bodies.
2. Statement of the Prior Art
Flexible bodies, such as those used as sonar receivers and for conducting seismic surveys for the purposes of oil and gas exploration, have two long-standing problems. One is the inherent cost, complexity and unreliability of conventional electromechanical heading and depth sensors used for shape estimation. Second is the difficulty and expense of handling that class of these devices typically mounted external to the arrays. In addition, accurate mathematical processing of array data is often compromised by the uncertain geometry of the towed array. In surface ships, for example, waves induce heave and surge motions in the towing vessel which propagate down the towing cable to the array itself. On the other hand in submarines, underwater currents, temperature, and density variations may also cause deformation of the array. Even when such environmental factors are minimal, any change in the course of the towing vessel necessitates waiting from ten minutes to more than an hour while the array becomes sufficiently straight and quiescent. The duration of this settling period depends upon the geometry of the array and its towing cable, local environmental conditions, speed of the towing vessel, and the severity of its maneuvers. Waiting for stability and/or quiescence is costly as well as time-consuming. Therefore, it is important to be able to perform real-time shape sensing with shape sensing of such arrays.
In marine seismic survey applications, up to twelve parallel towed arrays (i.e., "streamers") of approximately five kilometers each are deployed. To control the depth of the streamers, multiple leveling devices, or birds, are attached to the streamers as they are deployed. Because there are no means to control the displacement of the streamers parallel to the water surface, magnetic heading sensors (e.g., compasses) are incorporated into the birds to infer the streamer shape. This shape estimation scheme requires a vast amount of heading sensor data to be acquired during the survey, at the expense of collecting seismic acoustic data. and adds complexity to the deployment and retrieval operations.
Shorter arrays, such as the United States Navy's most advanced towed bodies (e.g., TB-29). are typically equipped with two environmental sensor modules located near the forward and aft positions of the towed array. Each module contains a total of ten sensors for measurements of depth, heading, roll, and tension of the towed array. There are also an additional three heading, depth, and roll sensors located in each of the eight aft acoustic modules. A vast amount of sensor data is used in conjunction with complex hydrodynamic mathematical models of flexible body behavior to infer shapes of the array. As a result, the primary disadvantage of such arrays are their relatively high cost and complexity, part of which is due to these sensors. It would, therefore, be desirable to provide a less expensive, far simpler shape determination scheme.
Considerable research and development has been directed in recent years towards the application of fiber optic technology in ocean engineering applications. Fiber optic technology has been tested in a wide variety of uses, including data transmission, secure communications, hydrophones, magnetometers, strain and temperature sensors, and guided munitions.
In addition to data transmission applications, fiber optic Bragg gratings have been used as low-cost, high-volume sensors for structural health monitoring in a variety of environments, including bridges, buildings, highways, aircraft, ships and spacecraft. This invention extends this technology to the realm of highly flexible structures such as cables, seismic streamers, and other types of arrays. Accordingly, it would also be desirable to provide a relatively inexpensive, simple shape determination scheme using fiber optic technology.
Fiber optic technology has also been widely exploited for ocean surveying acoustic arrays. Many advanced commercial and military systems incorporate uses of fiber optics- These arrays transmit data via multi-mode fiber optic busses to reduce the complexity, weight, and cost of analog multi-strand cables. Optical fiber-based hydrophones are also used in some applications for enhanced performance and reduced cost. Based on these trends towards the widespread adoption of fiber optic technology in acoustic arrays, it is desirable to leverage such existing use of optical fibers to provide a retrofitable, high-performance, array shape measurement technique at low cost.
In this regard, it would be desirable to provide inexpensive, simple fiber optic shape determination systems and methods for flexible bodies and other applications (e.g., in-flight refueling, remotely piloted vehicles, mining, construction, and other uses of flexible boring machines, and placing and positioning of moored navigational buoys).